I have a friend who is finally granted with the green card through employment, one month ago. Although he had an intension to work for the sponsor when he submitted I485 (5 months ago), he recently feels that he may want to change his job because of health condition (severe headache, he had a brain stroke 8 months ago). Also he received a good offer from another company (same kind of job with current) today.
I read a couple of articles that USCIS may deactivate the granted GC if he changes job right after the grant.

1)Is there any possibility that USCIS will accept his job change without deactivating, in the situation above?
2)How will USCIS know his job change anyways if he does not move (no address change) and the current employer does not claim for GC deactivation?